1. Field of the Invention
This invention relates to a radio receiver, and is applicable for example, to a digital cellular system transmitting and receiving voice signals by coding them.
2. Description of the Related Art
Conventionally, a digital cellular system, a kind of radiotelephony, is designed to allow a plurality of terminals to simultaneously use one channel through a time division multiplex approach by coding voice signals for transmitting and receiving.
That is, when it is turned on, such a type of terminal sequentially scans received frequencies in a predetermined number of channels, for example, 124 channels, and detects FCCH (frequency correction channel) which is inserted in the channel at a predetermined cycle (typically, consisting of 10 or 11 frames) in the sequence of stronger field strength, to recognize the channel including the FCCH as control channel.
Then, the terminal detects and receives a control channel assigned to the area to which it belongs.
The control channel is designed to form a time slot for the transmission of various information, whereby, in the digital cellular system, each terminal receives the control channel in order to receive information such as information on a base station sending the control channel, information on an adjacent base station, and information for calling the terminal.
To this end, the terminal corrects the processing timing based on this FCCH, and roughly detects timing for sending out necessary information.
Here, FCCH is a synchronizing signal assigned with a bit pattern in which, when it is decoded, data of value "0" continues for a predetermined number of bits. In the digital cellular system, the bit pattern is differential coded and then GMSK (Gaussian filtered minimum shift keying) modulated for transmission.
Thus, as shown in FIG. 1, the FCCH is transmitted as a combined wave of I and Q signals the signal level of which varies in a sine wave with a phase difference of 90.degree.. In a control channel, a carrier frequency offsets by +67.7 [kHz] while sending out the FCCH.
Thus, the digital cellular system is arranged to detect the timing of FCCH by extracting signal components of the FCCH from a received signal with a band pass filter, and to roughly synchronize the entire operation with the control channel based on the detection result of the timing.
In such case where the signal components of FCCH is extracted by using the band pass filter, the narrower the band width of the band pass filter is, the higher the detection accuracy is improved.
However, if the band width is made narrower, the response characteristics of the band pass filter tend to deteriorate.
On the other hand, such a digital cellular system is characterized by that the duration of FCCH is as short as 550 [usec] so that, if the response characteristics of the band pass filter deteriorate, it becomes difficult to find the FCCH.
In addition, in the digital cellular system, there may be a case where the frequency is displaced by doppler shift, or where a carrier frequency has the same frequency spectrum as the offset FCCH in a data other than the FCCH so that, when the band pass filter is used, there is such a problem that the timing of FCCH is erroneously detected or cannot be detected at all.
Thus, if the timing of FCCH is erroneously detected or cannot detected at all, detection is tried again for FCCH which is repeatedly transmitted at a predetermined cycle so that, in the digital cellular system, time is required until a call is made possible.
On the other hand, there is a method to detect FCCH from a decoded data stream.
That is, if the FCCH is formed by a data stream of value "0", it can be detected by detecting the correlation between a demodulated data stream and a data stream with consecutive "0" values.
However, in such a digital cellular system, the result of decoding may be mistaken because of noise or fading in addition to the doppler shift.
Particularly, for the noise level, there may be a case where Eb/NO (Eb: transmission energy per bit, NO: power intensity of noise) deteriorates to 10 [dB] or less. In such a case, the demodulated data stream has a higher rate of error so that the FCCH cannot be properly detected.
On the other hand, an approach may be considered in which I and Q signals are generated by orthogonally detecting the received signal, the correlation between the I and Q signals being detected by utilizing the fact that they have phase difference of 90.degree. in the FCCH, thereby detecting the FCCH.
However, because, as described above, there may be a case where it has high correlation between other than sine wave in a slot other than the FCCH, erroneous detection may be caused even if the correlation between the I and Q signals is detected.